Direct phase measurement may be used as a method for quantitatively evaluating line images produced in interferometric methods, or on projection of line patterns on objectives, or in moire methods. These methods serve to measure the optical path, or measuring the changes in the optical path caused by shifting or deformation of light dispersing objects, or changes in the refractive index of transparent objects. Projection or moire methods serve inter alia for ascertaining three-dimensional forms of objects, or the change thereof.
There has already been a proposal to evaluate line images on the surface of a three-dimensional body using phase shift technology. Such a method is described in the German Patent Publication 3,723,555 A, but limited, however, to projection and moire methods. The content of this prior publication is referred to expressly herein. In the known method, at least three phase-shifted images are supplied to a computer for evaluation. In order to ensure complete and automatic ascertainment of the three-dimensional configuration of the surface of the body, that is to say in order to ascertain all three-dimensional coordinates for each point of the surface of the body, it is mandatory to input at least three phase-shifted images to the computer and to evaluate the same, since there are three unknowns in the equation for the measured intensity of each image point: EQU I=a(X)(1+m(x) cos .phi.)
wherein:
I=intensity (measured), PA1 a=background brightness, PA1 m=contrast and PA1 .phi.=phase shift (quantity being sought). PA1 d=mean speckle diameter, PA1 L=wave length of the radiation, PA1 z=image distance (distance of the main plane of the image forming optical system from the image plane), and PA1 D=diameter of the image forming optical system (or of the objective from the image forming optical system).
Since it is only the intensity which may be measured, the above-noted equation will be seen, as previously mentioned above, to contain three unknowns. In order to ascertain the phase shift being sought, it is thus necessary to have three equations, this being accomplished by producing three phase-shifted images. Details are described in the above-mentioned German Patent Application Publication 3,723,555 A, the content of which is expressly referred to herein, as previously mentioned.
In the prior art method, it is possible for the three-dimensional surface of a body to be computed even with only one single shot or picture thereof, if the additional further information is input to the computer. In practice, however, it is frequently desirable, or sometimes even mandatory, to ascertain the three dimensional form of the surface of a body automatically without the necessity for inputting additional information. In such a case, when using the known method, three shots or pictures have to be produced. During the time elapsing between the production of these shots or pictures, the surface of the body may undergo modification. This may consequently lead to excessively inaccurate or even useless results. Even more particularly, in the case of vibration analysis, the form of the surface must be able to be ascertained by producing a single shot or picture.
It is an object of this invention to provide a method for the direct phase measurement of radiation and an apparatus for performing such a method and with which method and apparatus complete phase measurement is possible with a single shot or picture.